Control of the lower critical solution temperature—type cononsolvency properties of poly(N-isopropylacrylamide) in water—dioxane mixtures through copolymerisation with acrylamide

The behaviour of the homopolymers poly(N-isopropylacrylamide) (PNIPAM), polyacrylamide (PAM) and random copolymers of N-isopropylacrylamide (NIPAM) with acrylamide (AM) was studied by turbidimetry and viscometry in mixtures of water with dioxane. It was found that the well-known lower critical solution temperature-type cononsolvency properties of PNIPAM in water-dioxane mixtures, observed in the water-rich region, can be effectively controlled by copolymerisation of NIPAM with AM. Thus, the cononsolvency properties of the copolymers in water-dioxane mixtures are shifted to higher temperatures and restricted within a narrower solvent composition region as the acrylamide content of the copolymers increases. A significant decrease of the reduced viscosity of the systems exhibiting phase separation properties was observed upon heating, indicative of the collapse of the (co)polymer chains as temperature approaches the corresponding cloud point temperature. Furthermore, when temperature is fixed close to the cloud point temperature, the reduced viscosity decreases with increasing the volume fraction of dioxane, φ, as far as the solvent mixtures are rich in water. On the contrary, the reduced viscosity of PNIPAM in dioxane-rich mixtures is found significantly higher, indicative of an expansion of the polymer chain, as compared to the reduced viscosity of this polymer in the two pure solvents.     

Spectroscopic studies of polymer electrolytes based on poly(N-ethylethylenimine) and poly(N-methylethylenimine)

Poly(ethylethylenimine), PEEI, was prepared from poly(ethylenimine) by reductive alkylation with acetaldehyde. Samples of PEEI and poly(methylenimine), PMEI, complexed with LiCF₃SO₃ were prepared and characterized using differential scanning calorimetry and FT-IR. Small differences in the room temperature spectra of the two complexes were noted; these differences were due to the presence of a CH₂ group in the side chain of PEEI. The predominant form of cation-anion interactions was a contact ion pair. As the samples were heated, a transition from ion pairs to “free” ions was observed, with most of the change occurring between 140 and 150 °C in both PEEI and PMEI complexes. Thermal cycling established that the transition was irreversible in the time frame of the cycling experiments. Two-dimensional correlation spectroscopy did not show any significant intensity or frequency changes in bands sensitive to cation-polymer interactions during any heating or cooling cycle.     

Thermo- and pH-sensitive dendrimer derivatives with a shell of poly(N,N-dimethylaminoethyl methacrylate) and study of their controlled drug release behavior

Novel dendrimer derivatives combining the temperature- and pH-sensitivities are synthesized. At first, polyamidoamine (PAMAM) dendrimers with generations 1-5 are synthesized by the reaction of ethylenediamine with methyl acrylate, and then the dendrimers are acylated by chloroacetyl chloride to obtain PAMAM-Cl, which can act as a macroinitiator for further synthesizing functional dendrimers. For fulfilling this goal, the polymers consisting of a dendritic PAMAM core and poly(N,N-dimethylaminoethyl methacrylate) (PDMA) shell are synthesized by atom transfer radical polymerization (ATRP). Their macromolecular structures are characterized by FTIR, ¹H NMR, DSC and particle size analyses, and their aqueous solutions are inspected by UV spectroscopy for understanding their thermo- and pH-sensitivities. The results show that novel dendrimer derivatives exhibit clearly thermo- and pH-respondings in accordance with the change of the environment. Using chlorambucil (CLB) as a model drug, the behaviors of the controlled drug release from polymers with different average graft length of PDMA are studied. The results indicate that the rate of the drug release can be effectively controlled by the pH value.     

Influence of high-pressure on cononsolvency of poly(N-isopropylacrylamide) nanogels in water/methanol mixtures

We show that the temperature-induced collapse of poly(N-isopropylacrylamide) (PNiPAm) nanogels in water/methanol mixtures can be reversed by excess hydrostatic pressure. Small angle X-ray scattering (SAXS) results reveal that first a swollen surface layer is established and then the particles swell homogeneously. A threshold pressure needed for rewelling fully collapsed nanogels indicates that hydrophobic interactions inside the nanogel have to be compensated to form a swollen surface layer. The size change is related to a change in polymer solvation detected by infrared (IR) spectroscopy. Pressure favours polymer/water hydrogen bonds to the cost of methanol/polymer bonds so that water is enriched inside the nanogel.     

Thermosensitive poly(N-isopropylacrylamide) nanocapsules with controlled permeability

In this paper, novel thermosensitive poly(N-isopropylacrylamide) (PNIPAM) nanocapsules with temperature-tunable diameter and permeability are reported. Firstly, the core-shell composite microparticles were synthesized by precipitation polymerization with isothiocyanate fluorescein (FITC) entrapped SiO₂ as core and cross-linked PNIPAM as shell. Then, the SiO₂ core was etched by hydrofluoric acid at certain condition and the pre-trapped FITC molecules remained within the inner cavity. The FITC release profile and TEM studies clearly indicate that the release behavior of FITC could be controlled effectively by the external temperature. Above the LCST of PNIPAM (32 °C), the dehydrated PNIPAM shell inhibited the release of FITC from the internal cavity while below its LCST, the fluorophore could permeate the swollen shell easily.     

Thermo- and pH-sensitivity of aqueous poly(N-vinylpyrrolidone) solutions in the presence of organic acids

The phase transition in poly(N-vinylpyrrolidone) (PVP) aqueous solutions is shown to occur at heating upon addition of organic acids such as isobutyric, isovaleric, and, especially, trichloroacetic (TCA) ones. The cloud point temperature (T_c) of PVP solutions drops from 70 to 6 °C when the TCA concentration rises from 0.2 to 0.3 mol/l. A decrease in T_c is even more drastic when HCl is also added though HCl addition to the system without TCA does not result in phase separation. These phenomena are explained by the reversible coordination between the non-ionized form of TCA and PVP units via hydrogen bonding. An increase in the medium acidity depresses TCA dissociation, resulting in an increase in PVP-TCA associate concentration. Calculations based on the pK_a values of TCA confirm this suggestion. The similar behavior is observed with poly(N-vinylcaprolactam) systems. The amount of TCA bound to PVP has been determined by means of separation of the precipitate by centrifugation at temperatures above T_c and subsequent titration of TCA in the polymer with NaOH. It is shown that the precipitate contains one TCA molecule per 3-6 VP units, this value decreasing down to 1.25-2 upon HCl addition to the system.     

Degradation and geometric isomerization of poly(N-propargylamides)

The stability of several poly(N-propargylamides) was investigated in solution and in solid state on the basis of molecular weight change with time, and further their thermal stability was investigated by TGA. When the stability of poly(N-propargylamides) with varying pendent groups was compared, polymers with pendent groups of moderate size showed the highest stability in solution. Too short and too bulky pendent groups were not favorable for the stability of polymers. When poly(N-propargylheptanamide) (poly(6)) was stored in THF as solution at −20 °C in the absence of oxygen in dark, its degradation rate was the lowest. The degradation rate of poly(6) depended on the solvents used, which may be related to different solubility of oxygen in these solvents. Polymers with high cis contents degraded faster than polymers with low cis contents did. Addition of TEMPO and DPPH into the poly(6)/THF solution more or less depressed the degradation of poly(6). The degradation of polymer main chain in solution was always accompanied by the decrease of cis content, i.e. geometric isomerization from cis- to trans-structure. When the polymers were stored in the solid state at −20 °C, the polymers having alkyl pendent groups with moderate length were more stable than those with bulky pendent groups. Geometric isomerization occurred along with degradation in the solid state as well.     

Dynamic mechanical and tensile properties of poly(N-vinyl pyrrolidone)-poly(ethylene glycol) blends

Mechanical properties of miscible blends of high molecular weight poly(N-vinyl pyrrolidone) (PVP) with a short-chain, liquid poly(ethylene glycol) (PEG) of molecular weight 400 g/mol have been examined as a function of PVP-PEG composition and degree of hydration. The small-strain behavior in the linear elastic region has been evaluated with the dynamic mechanical analysis and compared with the viscoelastic behavior of PVP-PEG blends under large strains in the course of uniaxial drawing to fracture and under cyclic extension. A strong decoupling between the small-strain and the large strain properties of the blends has been observed, indicative of a pronounced deviation from rubber elasticity in the behavior of the blends. This deviation, also seen on tensile tests under fast drawing, is attributed to the peculiar phase behavior of the blends and the molecular mechanism of PVP-PEG interaction. Nevertheless, for the PVP blend with 36% PEG, under comparatively low extension rates, the reversible contribution to the total work of deformation up to ε=300% has been found to be maximum at around 70%, while the blends containing 31 and 41% PEG behave rather as an elastic-plastic solid and a viscoelastic liquid, respectively.     

Electropolymerization of 3,4-ethylenedioxythiophene, N-ethylcarbazole and their mixtures in aqueous micellar solution

Electropolymerization by cyclic voltammetry and chronoamperometry of N-ethylcarbazole (ETCZ) and 3,4-ethylenedioxythiophene (EDOT) was carried out in water-methanol (v/v: 1/3 0.01 M sodium dodecylsulfate (SDS) and 1.25 M perchloric acid on platinum button electrodes. Methanol improves the ethylcarbazole solubility and allows a well defined polymer growth on the working electrode. The SDS makes easier the ETCZ and EDOT electropolymerization. Indeed, the presence of micelles decreases the monomer oxidation potential and induces an acceleration of the polymerization. ETCZ and EDOT have similar monomer oxidation potentials in this medium. Moreover their polymers show a better stability than the polymers obtained without surfactants. Therefore, electropolymerization mixtures of these monomers was carried out with different ratios (v/v, 70/30, 50/50 and 30/70) in order to prepare copolymers and the resulting products were characterized by electrochemistry, IR and UV-visible spectroscopy and SEM.     

Characterization of electrospun poly(N-isopropyl acrylamide) fibers

Poly(N-isopropyl acrylamide) (pNIPAM) is an interesting material in that it shows a thermoresponsive behavior around 32 °C in aqueous solutions. This behavior mimics that of many proteins in solution and as a result, many researchers have studied pNIPAM as a model for protein behavior. Yet, little is known about the processability of pNIPAM into three-dimensional matrices and whether such processing affects polymer conformation. In this work, 3D fibrous mats of pNIPAM were prepared by electrospinning from three different solvents and the resulting morphologies evaluated. Additionally, electrospun pNIPAM was evaluated with polarized Raman and infrared spectroscopies and compared against the spectra of the bulk material. It was found that the electrospinning process did not alter the polymer structure or morphology.     

Thermosensitive poly(allylamine)-g-poly(N-isopropylacrylamide): synthesis, phase separation and particle formation

Grafting of poly(N-isopropylacrylamide) (PNIPAAm) having carboxylic groups at one end onto poly(allylamine) (PAH) in the presence of water soluble carbodiimide has yielded PAH-g-PNIPAAm copolymers with grafting ratios of 50, 29 and 18, respectively. These thermosensitive copolymers exhibit a lower critical solution temperature (LCST) at 34 °C at a temperature increase cycle regardless of their grafting ratios, a temperature identical to that of PNIPAAm-COOH oligomers. Temperature cycling reveals completely reversible polymer aggregation and dissolution above and below the LCST, respectively. Much smaller particle sizes are observed by scanning force microscopy and transmission electron microscopy compared to dynamic light scattering. A porous sphere model is suggested to depict the structure of the particles formed above the LCST, by which the dependence of the particle sizes on their grafting ratios is interpreted taking into account the surface tension and the spatial aggregation distance. Finally, to demonstrate the capability of the copolymers being used as thermosensitive polyelectrolytes, assembly onto multilayers is conducted and the increase of layer thickness is confirmed by small angle X-ray scattering and ellipsometry characterizations.     

Modification of poly(N-vinyl-carbazole) thin films by bromine doping

Poly(N-vinyl-carbazole) (PVK) thin films doped with bromine has been studied by scanning electron microscopy, X-ray diffraction, infrared absorption, X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), optical transmission (visible, near ultra violet) and conductivity measurements. The polymer has been doped at room temperature and at 373 K. It is shown by ESR, XPS and optical measurements that a charge transfer complex (CT-complex) is formed between PVK and Br. However, if some bromine acts as dopant of the polymer there is another bromine contribution, which corresponds to bromine covalently bonded to PVK and some only adsorbed. It is also shown by ESR that there is not only polymer doping by bromine but also some partial polymer degradation. Therefore, it can be said that the optimum doping condition of PVK thin films with bromine has been shown to be room temperature post-doping.     

Reentrant conformation transition in poly(N,N-dimethylacrylamide) hydrogels in water-organic solvent mixtures

Conformational changes in poly(N,N-dimethylacrylamide) (PDMA) networks swollen in aqueous solutions of organic solvents are studied both experimentally and theoretically. PDMA hydrogels of various charge densities were prepared by free-radical crosslinking copolymerization. Swelling behavior of the hydrogels was investigated in aqueous organic solvent mixtures as functions of solvent species and the concentration. With increasing volume fraction ? of acetone, tetrahydrofuran, or 1,4-dioxane in the aqueous solution, PDMA hydrogels exhibit reentrant conformation transition. During this transition, the gel first deswells in the range of ? between 0.4 and 0.9, and then rapidly reswells if ? is monotonically increased. The reswelling of the collapsed PDMA gel occurs in a narrow of ? above ?=0.97. It was shown that the reentrant transition in PDMA gels requires moderate hydrogen bonding organic solvents, so that the hydrophobic interactions between PDMA and the organic solvent dominate the swelling process. The results were interpreted using the theory of equilibrium swelling. The interaction parameters in the gel system as well as the partition parameter of the organic solvent between the gel and the solution phases were calculated.     

H NMR study of temperature-induced phase transitions in D2O solutions of poly(N-isopropylmethacrylamide)/poly(N-isopropylacrylamide) mixtures and random copolymers

¹H NMR spectroscopy was used to investigate temperature-induced phase transitions in D₂O solutions of poly(N-isopropylmethacrylamide) (PIPMAm)/poly(N-isopropylacrylamide) (PIPAAm) mixtures and P(IPMAm/IPAAm) random copolymers of various composition on molecular level. While two phase transitions were detected for PIPMAm/PIPAAm mixtures, only single phase transition was found for P(IPMAm/IPAAm) copolymers. The phase transition temperatures of PIPAAm component (appears at lower temperatures) are not affected by the presence of PIPMAm in the mixtures; on the other hand, the temperatures of the phase transition of PIPMAm component (appears at higher temperatures) are affected by the phase separation of the PIPAAm component and depend on concentration of the solution. For P(IPMAm/IPAAm) random copolymers, a departure from the linear dependence of the transition temperatures on the copolymer composition was found for a sample with 75 mol% of IPMAm monomeric units.     

Phase diagram of a cellulose solvent: N-methylmorpholine-N-oxide-water mixtures

The phase diagram of the N-methylmorpholine-N-oxide-H₂O mixtures from 0 to 100% has been determined. Three crystalline hydrates have been identified, the already known monohydrate, a dihydrate and a hydrate composed of 8 water molecules per NMMO. The melting temperature of the 8H₂O-NMMO hydrate is −47 °C with a melting enthalpy of about 80 J/g. The region between 25 and 55% of water does not show any crystallisation, but a glass transition around −60 to −100 °C.     

Synthesis of thermosensitive poly(N-alkylacrylamide) gels and core-shell type gels

When the poly(acrylic acid) (PAA) gel-1,8-diazabicyclo-[5,4,0]-7-undecene salt (DAA) was placed in N-methyl-2-pyrrolidone containing an excess of alkylamine and triphenylphosphine, selective amidation took place from the outside to give the corresponding poly(N-alkylacrylamide) gel containing a C3 alkyl chain through a DAA-poly(N-alkylacrylamide) type gel capsule consisting of a hydrophilic unreacted core part and an amidated shell layer. The amidation proceeded by a reaction mechanism similar to the unreacted-core model. Thermal properties of the resulting poly(N-alkylacrylamide) gels such as deswelling behavior and equilibrium swelling ratio in water as a function of temperature were measured. The release of methyl orange from a poly(N-alkylacrylamide) gel and the gel capsule was also examined. PAA-poly(N-alkylacrylamide) type gel capsules containing a PAA core part and thermosensitive poly(N-alkylacrylamide) shell layer, prepared by the neutralization of DAA-poly(N-alkylacrylamide) type gel capsules, showed on-off chemical release characteristics in response to stepwise temperature changes across the LCST.     

Interactions in miscible blends and complexes of poly(N-acryloylmorpholine) with poly(p-vinylphenol)

Poly(p-vinylphenol) (PVPh) and poly(N-acryloylmorpholine) (PAcM) form interpolymer complexes in ethanol/water (1:1) solution. However, only ordinary blends are obtained from dimethylformamide solution. Each of the complexes and ordinary blends shows one composition-dependent glass transition temperature, indicating its single-phase nature. Fourier transform infrared spectroscopy and ¹³C solid-state nuclear magnetic resonance spectroscopy reveal the existence of hydrogen-bonding interactions between the hydroxyl groups of PVPh and the carbonyl groups as well as the ether oxygen of PAcM in the blends and complexes. In addition, X-ray photoelectron spectroscopy shows that the nitrogen atoms in PAcM are also involved in hydrogen-bonding interactions. Measurements of proton spin-lattice relaxation time in the rotating frame, T_1ρ(H), reveal that each of the complexes and ordinary miscible blends has one composition-dependent T_1ρ(H), indicating an intimate mixing on a scale of about 1.5 nm. The blends show a higher degree of surface enrichment of PVPh than the complexes.     

Thermo-, and pH dual-responsive poly(N-vinylimidazole): Preparation,characterization and its switchable catalytic activity

A monomer, 2-(isobutyramido)-3-methylbutyl methacrylate (IMMA) was synthesized through a two-step reaction. When a few of IMMA (less than 4 mol%) was copolymerized with N-vinylimidazole (VIm) under free radical polymerization condition, water-soluble P(VIm-co-IMMA) copolymers were obtained. Their structural information was verified and interpreted from ¹H NMR, FTIR and GPC. Kinetic analyses from ¹H NMR demonstrated that one-batch addition of IMMA into the polymerization system led to an inhomogeneous distribution of IMMA units in the copolymers, whereas homogeneous distribution of IMMA units in the copolymers could be obtained through the portion-wise addition of IMMA monomer. The thermal properties of such copolymers were measured by DSC. Compared with PVIm homopolymer, the few IMMA units in the P(VIm-co-IMMA) copolymer had little influence on the T_g values. The obtained P(VIm-co-IMMA) copolymers were thermoresponsive in water, and their phase transition temperatures could be efficiently raised through reducing the IMMA content in the copolymers, raising the addition times of IMMA monomers or lowering the pH of media. Dynamic light scattering analysis showed that unlike the traditional thermoresponsive linear polymers, obvious size shrinkage around the phase transition temperature could not be observed in such P(VIm-co-IMMA) copolymers. Such copolymers could be used as smart organocatalysts in the hydrolysis of p-nitrophenyl acetate. Below the phase transition temperature the reaction rate followed the Arrhenius law, but above the phase transition temperature the reaction rate increased much slower than the prediction from the Arrhenius law. Moreover, the catalytic transition temperature could be tuned through utilizing the P(VIm-co-IMMA) copolymers with different phase transition temperature. The mechanism was discussed accordingly.     

Direct UV photocrosslinking of poly(N-vinyl-2-pyrrolidone) (PVP) to produce hydrogels

Hydrogels for biomedical purposes, made from synthetic polymers as starting materials and free of co-adjuvant molecules, have been produced almost exclusively by high-energy radiative processes. On the other hand, UV photocrosslinking of such materials has been used in conjunction of monomers and/or photoinitiators. This work was addressed to the analysis of poly(N-vinyl-2-pyrrolidone) (PVP) submitted to direct photocrosslinking in aqueous solution, using low pressure Hg lamp (λ_em=254 nm). The process efficiency was evaluated, and the properties of the hydrogel formed were determined. The product thus formed has similar micro- and macroscopic properties, as compared to hydrogels produced by high-energy radiation and presents no cytotoxicity. These results demonstrated the viability of using this method as a versatile alternative to hydrogel production, broadening the possibility of its production where high-energy radiation facilities are not available.     

Effect of hydrophobic polystyrene microphases on temperature-responsive behavior of poly(N-isopropylacrylamide) hydrogels

Reversible addition-fragmentation chain transfer polymerization was employed to prepare the crosslinked poly(N-isopropylacrylamide)-graft-polystyrene networks (PNIPAAm-g-PS). Due to the immiscibility of PNIPAAm with PS, the crosslinked PNIPAAm-g-PS copolymers displayed the microphase-separated morphology. While the PNIPAAm-g-PS copolymer networks were subjected to the swelling experiments, it is found that the PS block-containing PNIPAAm hydrogels significantly exhibited faster response to the external temperature changes according to swelling, deswelling, and reswelling experiments than the conventional PNIPAAm hydrogels. The improved thermo-responsive properties of hydrogels have been interpreted on the basis of the formation of the specific microphase-separated morphology in the hydrogels, i.e., the PS blocks pendent from the crosslinked PNIPAAm networks were self-assembled into the highly hydrophobic nanodomains, which behave as the microporogens and thus promote the contact of PNIPAAm chains and water. The self-organized morphology in the hydrogels was further confirmed by photon correlation spectroscopy (PCS). The PCS shows that the linear model block copolymers of PNIPAAm-g-PS networks were self-organized into micelle structures, i.e., the PS domains constitute the hydrophobic nanodomains in PNIPAAm-g-PS networks.